Switching regulators (switching mode power converters), including ripple regulators, are commonly used because of their characteristic of high efficiency and high power density resulting from smaller magnetic, capacitive and heat sink components.
Synchronous buck power stages are a specific example of switching regulators that use two power switches such as power MOSFET devices. A high-side switch selectively couples an inductor to a positive power supply while a low-side switch selectively couples the inductor to ground. A pulse width modulation (PWM) control circuit is used to control the high-side and low-side switches. Synchronous buck regulators provide high efficiency when low ON-resistance power switches such as MOSFET devices are used.
A buck converter is an example of a commonly used switching mode power converter. A typical buck converter includes high-side and low-side power transistors (e.g., switching transistors, or transistor switches) connected between a supply voltage (input voltage) and ground, as well as an output inductor and an output filter capacitor connected in series between an intermediate node (switching node, LX node) arranged between the high-side and low-side switching transistors, and ground. The switching transistors may be FET devices, such as MOSFETs, for example.
In this arrangement, the switching transistors are alternatively activated (e.g., placed in the ON state), providing energy to a load, through the output inductor and across the output filter capacitor. The output voltage is regulated by a controller (e.g., control circuit), which determines the ON and OFF cycles of the switching transistors. Therein, the switching transistors are alternatively placed in the ON state since placing the switching transistors simultaneously in the ON state would effectively create a short circuit across the input voltage.
The transitions when both switching transistors are in the OFF state for a brief period of time are commonly referred to as dead times. During these periods, the output inductor produces a freewheeling current that flows through the body diode of the low-side switching transistor. The large forward voltage drop of the body diode results in high diode conduction and reverse recovery losses and adds to the thermal stress on the low-side switching transistor. As this process is repeated during every switching cycle, it also places a limitation of the maximum switching frequency of the switching mode power converter. This in turn prevents size reduction of the switching mode power converter, which would be a major benefit to end products such as portable equipment.
In addition, MOSFET parasitic body diode reverse recovery occurs during diode switching from the ON state to the OFF state since its stored minority charges must be removed. The minority charges may be removed either actively via negative current, or passively via recombination inside the device. All stored charge should be removed so that the depletion region can become big enough to block the reverse voltage.
The aforementioned condition occurs in synchronous DC/DC power converters every time that the low-side switching transistor is turned OFF and the high-side switching transistor is going to turn ON. In other words, the accumulated charge in the body diode of the low-side switching transistor needs to be removed each time the high-side switching transistor is turned ON.